I am wondering what the little recycling symbols that you see on some plastic items mean. How many different symbols are there?

A typical symbol is shown here. This one is from the lid of a kitchen container.

There are seven of these symbols currently, each one identifying a different type of plastic. The following list maps the numbers to the types of plastic:

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1 - Polyethylene terephthalate (PET)

2 - High-density polyethylene (HDPE)

3 - Polyvinyl chloride (PVC)

4 - Low-density polyethylene (LDPE)

5 - Polypropylene (PP)

6 - Polystyrene (PS)

7 - Other resins, like acrylonitrile butadine styrene (ABS)

PET is most commonly seen in plastic soda bottles and Mylar film. HDPE is what milk jugs are made of, and is also used in things like plastic gas cans. PVC is the base for any form of vinyl (from siding to seat covers), as well as many soft toys. LDPE is found in things like plastic food containers. The difference between HDPE and LDPE is that LDPE is softer, more flexible and melts at a lower temperature than HDPE. PP is a lightweight plastic and has a relatively high melting point -- it is used in things like luggage and plastic trim in cars, as well as food containers. PS is commonly known by the Styrofoam brand name and is used in everything from coffee cups to coolers.

These symbols only scratch the surface of the hundreds of different types of plastics around you. Think about it for a second and you can understand the variety of plastics in use today: nylon, polyurethane, polyurethane foam, all sorts of non-melting plastics used in cookware, epoxies, fiberglass, polycarbonates and so on. We've still got a way to go before they are all recyclable...